1. Technical Field
The present invention relates to a surface acoustic wave device using a quartz plate, and more particularly, to a surface acoustic wave device capable of improving a frequency control performance by reducing a capacitance ratio.
2. Related Art
Recently, surface acoustic wave (hereinafter, referred to as a SAW) devices have been widely used as parts for mobile communication terminals or built-in vehicle apparatuses. The SAW devices have been required to have a small size, a high Q value, and excellent frequency temperature characteristics.
As a SAW device stratifying the requirement, there is a SAW device using an ST cut quartz plate. The ST cut quartz plate is a name of a cut quart plate having a plane (XZ′ plane) which is formed by rotating an XZ plane by 42.75° counterclockwise from an Z crystalline axis about an X crystalline axis as a rotation axis, and the SAW device uses a SAW (hereinafter, referred to as an ST cut quartz SAW) which is a (P+SV) wave, that is, a so-called Rayleigh wave propagating in a direction of the X crystalline axis. The ST cut quartz SAW device may be widely applied to an SAW resonator used as an oscillation frequency element, an IF filer disposed between RF ports and ICs in a mobile communication terminal, or the like.
Since it is possible to efficiently use the reflection of the SAW, the size of the ST cut quartz SAW device can be reduced, and a device having a high Q value can be implemented. Now, an ST cut quartz SAW resonator shown in FIG. 12 is exemplified. The ST cut quartz SAW resonator has a structure where comb-like electrodes (hereinafter, referred to as an IDT) having a plurality of lines of electrode fingers which are alternately inserted are disposed on an ST cut quartz plate 101, and grating reflectors 103a and 103b reflecting the SAW are disposed at both sides of the IDTs 102. Since the ST cut quartz SAW is a wave propagating along a surface of the piezoelectric substrate, the ST cut quart SAW can be efficiently reflected on the grating reflectors 103a and 103b, so that the SAW energy can be sufficiently held within the IDTs 102. As a result, the device having a small size and a high Q value can be implemented.
In addition, as the important factors in use of the SAW device, there are frequency temperature characteristics. In the aforementioned ST cut quartz SAW, a first-order temperature coefficient of the frequency temperature characteristics is zero, and the characteristics are represented by a quadratic curve. Since the frequency variation can be greatly reduced by adjusting a peak temperature to a center of a usage temperature range, it is generally known that frequency stability thereof is excellent.
Although the first-order temperature coefficient is zero, a secondary temperature coefficient of the ST cut quartz SAW device has a relatively large value of −0.034 ppm/° C.2, so that there is a problem in that, if the usage temperature range is widened, the frequency variation increases extremely.
As a technique for solving the above problem, there are SAW devices disclosed in the article “Surface Skimming Bulk Wave, SSBW” (by Meirion Lewis, IEEE Ultrasonics Symp. Proc., pp. 744-752 (1977)) and JP-B-62-016050. As shown in FIGS. 13A and 13B, in the SAW device, a cut angle θ of a rotated Y-cut quartz plate is set to a vicinity of an angle rotated by −50° counterclockwise from a Z crystalline axis, and a propagation direction of the SAW is set to a direction (direction of a Z′ axis) perpendicular to the propagation direction of the SAW. In addition, the aforementioned cut angle can be represented with (0°, θ90°, 90°)=(0°, 40°, 90°). The SAW utilizes the IDT to excite the SH wave propagating along a portion directly underlying a surface of the piezoelectric substrate, and the vibration energy can be held within a portion directly underlying the electrode. In addition, the frequency temperature characteristics are represented with a quartic curve, and the frequency variation is very small in the usage temperature range, so that excellent temperature characteristics can be obtained.
However, since the SH wave is basically a wave attenuating into an inner portion of the plate, the reflection efficiency of the grating reflector for the SAW is low in comparison to the ST cut quartz wave propagating along the surface of the piezoelectric substrate. Therefore, there is a problem in that it is difficult to implement a SAW device having a small size and a high Q value. Although an application as a retardation line which the reflection of the SAW is not utilized for is disclosed in the aforementioned prior documents, any application of a device utilizing the reflection of the SAW is not disclosed. Therefore, there is a problem in that it is difficult to implement a practical oscillation frequency element or filter element.
In order to solve the above problem, a so-called multiple-pair IDT type SAW resonator is disclosed in JP-B-01-034411. As shown in FIG. 14, in the multiple-pair IDT type SAW resonator, a large number of pairs, that is, 800±200 pairs of IDTs 112 are disposed on a piezoelectric substrate 111 which has the cut angle θ of the rotated Y-cut quartz plate set to a vicinity of −50° and a propagation direction of the SAW set to a direction (a direction of the Z′ axis) perpendicular to the X crystalline axis. Accordingly, the SAW energy can be held by means of only the reflection of the IDT 112 without using any grating reflector, so that a multiple-pair IDT type SAW resonator having a high Q value can be implemented.
However, in comparison to the SAW resonator utilizing the grating resonator, the multiple-pair IDT type SAW resonator can not efficiently hold the energy, so that the number of IDT pairs increase up to 800±200 in order to obtain a high Q value. Therefore, the size thereof is larger than that of the ST cut quartz SAW resonator, there is a problem in that a recent requirement for a small size can not be satisfied.
In the SAW resonator disclosed in the aforementioned JP-B-01-034411, when a wavelength of the SAW excited in the is denoted by λ, the Q value can be increased by setting the electrode film thickness to 2% λ or more, preferably, 4% λ or less. In case of a resonance frequency of 200 MHz, the Q value is saturated at a vicinity of 4% λ, and at this time, the Q value is merely about 20000, which is substantially equal to that of the ST cut quartz SAW resonator. This is because the SAW is not sufficiently held within the surface of the piezoelectric substrate when the film thickness is in a range of 2% λ to 4% λ. Therefore, the reflection thereof can not efficiently used.
For these reasons, it is conceivable to provide a SAW device wherein IDTs made of Al or an alloy containing Al as a main component are disposed on a rotated Y-cut quartz plate having a cut angle θ set to a range of −64.0°<θ<−49.3°, preferably, −61.4°<θ<−51.1° rotated counterclockwise from a Z crystalline axis and a propagation direction of the surface acoustic wave set to a direction of 90°±5° with respect to an X crystalline axis and wherein an electrode film thickness H/λ of the IDT normalized with a wavelength of the SAW is set to 0.04<H/λ<0.12, preferably, 0.05<H/λ<0.10. According to the invention, the wave attenuating into the inner portion of the piezoelectric substrate is concentrated on the surface of the plate, so that the reflection of the SAW can be efficiently used. As a result, it is possible to implement an SAW device having a smaller size, a higher Q value, and more excellent frequency temperature characteristics than those of the ST cut quartz SAW device in the related art.
On the other hand, as an important factor for determining characteristics of the SAW device such as the SAW resonator and the SAW filter, there is a capacitance ratio γ. FIG. 15 shows an equivalent circuit diagram of an SAW resonator, and the capacitance ratio γ is represented by γ=C0/C1. As the capacitance ratio γ decreases, oscillation of the oscillating circuit using the SAW resonator can be further facilitated, so that it is possible to obtain a wide variable range of oscillation frequency and to widen a practical band width of the SAW filter.
The capacitance ratio γ may vary greatly according to an occupancy rate (hereinafter, referred to as a line occupancy rate mr) of an electrode finger width L to the electrode pitch (electrode finger width L+ electrode finger spacing S) of the IDT. Therefore, if the line occupancy rate mr of the IDT is not suitably selected, the capacitance ratio γ may be too large to obtain desired characteristics. However, since the capacitance ratio γ may not be conceived in the aforementioned conceivable SAW device, there is a need to scrutinize a relation between the capacitance ratio γ and the line occupancy rate mr in detail.
In addition, during a production process of the SAW device, it is difficult to accurately control the line occupancy rate mr, so that there occurs a deviation caused by production error or measurement error during the formation of the electrodes. If the line occupancy rate mr is not uniform, the frequency variation occurs, so that production yield is reduced. Therefore, the line occupancy rate mr is needed to be suitably selected in order to obtain excellent frequency control performance. However, the frequency control performance may not be conceived in the aforementioned conceivable SAW device, there is also a need to scrutinize a relation between the capacitance ratio γ and the frequency control performance in detail.
In addition, in the aforementioned conceivable SAW device, as a result of an electrical conduction test using the SAW resonator at a resonance frequency 433 MHz, the frequency variation during the electrical conduction further increases in comparison to a ST cut quartz SAW device in the related art. The reason is as follows. In the aforementioned conceivable SAW device, the electrode film thickness is larger than that of a ST cut quartz device in the related art. If the electrode film thickness is large, stress to a film during the formation of the film increases, so that there occurs stress migration that Al atoms migrates so as to alleviate the stress. Accordingly, frequency variation increases. In addition, if the electrode film thickness is reduced in order to avoid the frequency variation, the Q value is greatly lowered, so that there is a problem in that an insertion loss during the formation of a filter or the like increases.